Ultrasound Assessment of the Flexor Carpi Ulnaris for Early Detection of Muscle Atrophy in Critically Ill Patients: A Potential Indicator of ICU-Acquired Weakness

BackgroundIn critically ill patients admitted to the intensive care unit (ICU), rapid skeletal muscle atrophy frequently develops in the acute phase. This ICU-acquired weakness can significantly impair long-term physical function. Although the biceps brachii cross-sectional area (CSA) is commonly used to assess muscle atrophy, its ultrasound imaging can be technically challenging, and the flexor carpi ulnaris may offer a more accessible alternative. Therefore, this study aimed to investigate whether CSA changes of the flexor carpi ulnaris correlate with those of the biceps brachii in critically ill patients admitted to the ICU, as well as whether the flexor carpi ulnaris CSA reflects systemic muscle atrophy in the acute phase of the ICU stay. MethodsTwenty critically ill patients admitted to the ICU underwent serial ultrasound assessment of the biceps brachii and flexor carpi ulnaris CSAs on days 0, 5, 7, and 14 after admission. Longitudinal changes in CSA were analyzed using the Friedman and Wilcoxon signed-rank tests. Correlations between the biceps brachii and flexor carpi ulnaris were examined using Spearmans rank correlation, and structural equation modeling was applied to explore causal relationships between clinical variables and CSA changes. ResultsSignificant CSA reductions were observed in both the flexor carpi ulnaris (-20.6%) and biceps brachii (-16.3%) by day 14, and the relative CSA changes of the biceps brachii and flexor carpi ulnaris showed a moderate positive correlation ({rho} = 0.5489, p = 0.0122). Structural equation modeling analysis revealed that the biceps brachii CSA change had positive effect on that of the flexor carpi ulnaris ({beta} = 0.249, p = 0.0011). Moreover, body mass index was positively associated with the baseline flexor carpi ulnaris CSA ({beta} = 0.042, p = 0.0004). However, the baseline flexor carpi ulnaris CSA was not a significant predictor of subsequent CSA changes. ConclusionUltrasound measurement of the flexor carpi ulnaris CSA offers a practical alternative to that of the biceps brachii for early detection of muscle wasting in ICU patients. Given its anatomical accessibility and high sensitivity to early atrophic changes, it may serve as a feasible screening tool for ICU-acquired weakness and inform timely interventions.